The most worrisome infectious agents of bioterrorism are those that would be artificially disseminated as aerosols to the lungs where the immune responses are unique, primarily driven by an immunoregulatory program. Critical mediators of the immune response in this environment are alveolar macrophages (AM) and epithelial cells. Francisella tularensis (Ft), a targeted infectious agent of bioterrorism, is an intracellular pathogen which causes acute life-threatening disease, particularly when transmitted via aerosols. The eukaryotic and microbial factors mediating host cell recognition and response for Ft remain poorly defined and their definition will be required for the rational development of new therapies and vaccines. This competitive renewal of RP9 unites an established group of collaborative scientists from the Ohio State University, Medical College of Wisconsin and University of Cincinnati to focus on the molecular pathogenesis of pneumonic tularemia. Based on recently published work and preliminary data, our central hypothesis is that Ft's success as a human pathogen is linked to its ability to suppress and/or subvert important elements of the protective innate immune response during the early phase of infection. To test this hypothesis we will 1) further define the molecular mechanisms that mediate Ft entry, intracellular trafficking, antimicrobial responses and initiation of the inflammatory program both in vitro and in vivo;2) construct, validate, and probe global microbial and host genetic screening platforms to identify key determinants mediating Ftinduced immune suppression and/or subversion;and 3) use targeted genetic approaches to identify novel Ft virulence determinants focusing on bacterial transcriptional regulators and Ft and host factors involved in Fe metabolism, including those of the bacterial oxidative stress response. Collectively, these studies will provide important new insights into host-pathogen interactions by the most virulent Ft subspecies, and will use novel mutant derivatives and relevant model systems of infection. This RP will synergize the talents of its investigators to place itself in the most competitive position to make fundamental discoveries related to pathogenic mechanisms for Ft, and begin to translate these discoveries into product development.